Magnetic particles for purifying nucleic acids

ABSTRACT

Preparations of particles having a glass surface, wherein more than 75% by weight of these particles have a particle size between 0.5 μm and 15 μm and a glass surface which contains between 2 and 6 mole % zinc oxide, have proven to be particularly advantageous in processes for the purification of nucleic acids. This in particular results in an increased nucleic acid yield.

[0001] The application concerns a preparation of particles having aglass surface, a process for producing such a preparation and a processfor purifying nucleic acids with the aid of this preparation.

[0002] Nucleic acids have recently become more and more the focus ofinterest for medical diagnostics. Numerous detection methods have nowbeen developed in which the presence or absence of certain nucleic acidsis used as an indication for a disease. These include for example testsfor infectious organisms e.g. for viruses or bacteria in body fluids andalso the detection of mutations in genomic nucleic acids e.g. inoncology. However, nucleic acids are usually present at very lowconcentrations in the sample material. Hence various methods have beendeveloped for isolating nucleic acids from other sample components suchas proteins or other cellular components some of which interfere withthe subsequent detection methods. Some of these methods utilize captureprobes bound to solid phases that can hybridize with the nucleic acidsto be separated and retain these on the solid phase while the othersample components are removed. Such a method is described for example inEP-B-0 305 399. However, a disadvantage of these methods is that theyare each only suitable for purifying nucleic acids having a very specialnucleotide sequence.

[0003] A process for isolating nucleic acids with the aid of magneticparticles composed of cellulose and iron oxide is described in WO91/12079 in which the particle size is stated to be between 1 and 10 μm.These particles do not contain a glass surface and are only suitable foran isolation in which the nucleic acids are precipitated. However, theaggregation process also entraps many sample components which interferewith subsequent process steps.

[0004] EP-B-0389 063 proposes a process in which the sample is mixedwith a mixture of a chaotropic guanidinium salt and silica particles.Under these conditions the binding of the nucleic acids to the silicasurface is relatively independent of the sequence. The other samplecomponents can be removed by washing and the nucleic acids can besubsequently eluted.

[0005] Magnetic particles having an essentially pore-free glass surfaceare described in WO 96/41811 for the sequence-independent purificationof nucleic acids. The particles used in this case have a core whichpreferably contains magnetite as a magnetic material and they preferablyhave a particle size between 10 and 60 μm. Magnetite exhibits hardmagnetic properties in crystals larger than ca. 30 to 50 nm. Permanentmagnetism is induced by an external magnetic field. Particles havingsuch hard magnetic cores have the properties of a small permanent magnetafter their first exposure to an external magnetic field. In suspensionssuch particles attract one another and form larger units. Under theinfluence of an external field of gravity these large units sedimentmore rapidly than the individual particles. This is disadvantageoussince long periods of incubation require frequent redispersing.

[0006] Pigments are described in WO 96/41840 which have a glass surfacewith a thickness of at least 0.8 μm. Zinc compounds are also proposed asa glass forming component. Pigment particles are formed in this processhaving a particle size of preferably 2 to 20 μm.

[0007] It has now turned out that in the previously described processesfor the production of particles using a sol-gel process in which coreparticles having a specified size are coated with a gel and subsequentlya compression takes place to form a glass surface, a large proportion ofparticles are formed which do not contain core particles. Nucleic aciddetection methods carried out using such preparations either result inlarge losses of nucleic acids or the fines have to be laboriouslyseparated in order to increase the yield. The object of the presentinvention was to completely or partially improve the present state ofthe art and in particular to produce particles having a relativelynarrow particle size distribution and to further increase the yield innucleic acid purifications, or/and to provide particles for nucleic acidpurification which, even after exposure to an external magnetic field,only have a very low tendency to aggregate and sediment in agravitational field just as slowly as particles that have never beenexposed to a magnetic field.

[0008] The invention concerns a preparation containing particles havinga glass surface wherein more than 75% by weight of these particles havea particle size between 0.5 and 15 μm.

[0009] Further subject matters of the invention are a process forproducing a preparation of particles containing a core coated with a gellayer or a glass layer and a process for purifying nucleic acids withthe aid of the preparation according to the invention.

[0010] A further subject matter of the invention is a process forproducing particles and a preparation of particles having asuperparamagnetic core.

[0011] The invention additionally concerns a process for producingparticles and a preparation of particles having a magnetic andpreferably a soft magnetic metallic core.

[0012] Solid materials having a small diameter are referred to asparticles by a person skilled in the art. These particles preferablyhave an essentially spherical surface. However, platelet-shaped andfilamentary particles having the dimensions stated below are also to beunderstood as particles. In order to be particularly suitable forpurifying nucleic acids, it is desirable that the particles have a core(pigment part) which is preferably magnetic and is coated with a layerof glass. Such cores preferably contain metal oxides such as aluminiumoxide, iron oxide, chromium oxide, copper oxide, manganese oxide, leadoxide, tin oxide, titanium oxide, zinc oxide and zirconium oxide ormetals such as Fe, Cr, Ni or magnetic alloys. The composition of thiscore is less important for the function of the particles according tothe invention since the core is coated with a glass surface and hencethe core does not come into direct contact with the sample from which itis intended to isolate the nucleic acid. Such cores are commerciallyavailable. If the core contains Fe₃O₄ (magnetite) or Fe₂O₃ (maghemite)or Fe or Cr or Ni or magnetic alloys, then these cores are magnetic.

[0013] Suitable materials referred to as being soft magnetic are metalsbased on the pure elements Fe, Ni, Cr and alloys thereof preferablybased on Ni. Examples of such alloys are known under the name permalloy.They are composed of 70 to 80% Ni with additives of Cr, Cu and Mo.Particles consisting of magnetically soft material do not attract oneanother or only to a negligible extent in the absence of an externalmagnetic field.

[0014] Finely-dispersed metal powders are very reactive. There is a riskof self-ignition in air, they are pyxophoric. Hence it was verysurprising that such finely dispersed metal particles could be coatedwith a glass layer by a sol-gel process without significantly changingthe magnetic properties. Carbonyl iron powder is particularly preferablyused as a metal powder and types thereof that have been reduced in H₂have particularly favourable magnetic properties. Carbonyl iron whiskershave particularly favourable properties.

[0015] Metal powders preferably have a particle size between 10 nm and100 μm and particularly preferably between 200 nm and 8 μm.

[0016] A glass surface in the sense of the present invention is composedof an amorphous material containing silicon. In addition to siliconoxide the glass preferably contains one or several of the followingcomponents (in mole %):

[0017] B₂O₃ (0-30%), Al₂O₃ (0-20%), CaO (0-20%), BaO (0-10%), K₂O(0-20%), Na₂O (0-20%), MgO (0-18%), Pb₂O₃ (0-15%), ZnO (0-6%).

[0018] A number of other oxides can also be present in small amounts of0-5% such as e.g. Na₂O, Mn₂O₃, TiO₂, As₂O₃, Fe₂O₃, CuO, ZrO₂, CoO etc.Surfaces having a composition of SiO₂, B₂O₃, Al₂O₃, CaO, K₂O and ZnOhave proven to be particularly effective. Boron silicate glasses thatare particularly advantageous with regard to the yield of nucleic acidshave a zinc oxide content of 2-6, preferably of ca. 4 mole %. The glasslayer is particularly preferably composed of 68-79 mole % SiO₂, 15-5mole % B₂O₃, 6-2.5 mole % total amount of K₂O and Na₂O, 4-1 mole % CaO,8-2 mole % Al₂O₃, 6-2 mole % ZnO. Glasses are particularly preferred inthe sense of the invention which have been formed by the so-calledgel-sol process and subsequent drying and compression of the layer thatforms. The essential aspects of this process are known and have beendescribed for example in C. J. Brinker, G. W. Scherer “Sol Gelscience—The physics and chemistry of Sol Gel Processing”, Academic PressInc. 1990 and Sol-Gel Optics, Processing and Applications Lisa C. KleinEd. Kluwer Academic Publishers 1994 page 450 ff and in DE-A-1941191,DE-A-3719339, DE-A-4117041 and DE-A-4217432. In the gel-sol processalkoxides of network-forming components e.g. SiO₂, B₂O₃, Al₂O₃, TiO₂,ZrO₂ and ZnO are added together with oxides and salts of othercomponents e.g. in an alcoholic solution and hydrolysed.

[0019] The addition of water starts the hydrolysis of the startingcomponents. The reaction proceeds relatively rapidly since the alkaliions have a catalytic effect on the rate of hydrolysis of the silicicacid ester. After gel formation is completed, the gel can be dried andcompressed to form a glass by a thermal process.

[0020] The quantity ratio of sol to pigment has a considerable influenceon the yield of the magnetic pigment according to the invention. Theconstraints are that the amount of pigment has to be low enough to allowa material to form that can still be pumped and sprayed. If the amountof pigment is too low, the proportion of fine material e.g. non-magneticmaterial becomes too large and interferes. Quantity ratios of 10 to 45 gpigment/100 ml sol have been found to be expedient with regard to thepigment yield.

[0021] The slurry is preferably sprayed through a nozzle to form apowder and the aerosol is dried on a falling path. The nozzle ispreferably heated to accelerate the drying of the slurry. The nozzletemperature is preferably ca. 120 to 250° C. independent of the geometryof the nozzle. A compromise must be found between an adequate rate ofevaporation and avoidance of spattering.

[0022] The compression temperature should be selected to be as high aspossible with regard to yield. However, if it is too high, the particlesagglutinate and agglomerates form which should be removed by sieving.But addition of zinc to the layer surprisingly increases the meltingpoint and it is thus possible to use a higher compression temperature(between 710 and 800° C.). The after treatment in air leads to a loss ofthe magnetic properties if the temperature is too high which is whyexcessive temperatures should be avoided. When zinc is added it is alsopossible to use other temperatures in this case (preferably between 150and 250° C.).

[0023] Within the scope of the present invention it has turned out thatmagnetic cores can be used in the process described in WO 96/41811 whichare very much smaller. In particular it turned out that it is possibleto use cores on a nanoscale e.g. magnetite having a crystal size of lessthan 50 nm, preferably less than 30 nm. The lower limit of the core sizeresults from the handling properties of the cores and in particulartheir tendency to form aggregates. The cores are preferably larger than5 nm, particularly preferably larger than 7 nm. The magnetic propertiesof the nanoscale cores is referred to as superparamagnetic. Theparticles that are obtained sediment rapidly when exposed to an externalmagnetic field. After redispersion their sedimentation rate in agravitational field do not differ from their sedimentation rate in agravitational field before exposure to the external magnetic field. Thisis advantageous since it enables longer incubation times in suspensionwithout having to remix and resuspend.

[0024] In order to produce a preparation according to the invention apreparation of core particles in which more than 75% by weight of thecore particles have a particle size between somewhat less than 0.5 andsomewhat less than 15 μm is used in a sol/gel process. The coreparticles must be smaller than the glass-coated particles to the extentof the thickness of the glass layer. After the inventive process theglass layer will be between 5 nm and 1 μm thick depending on theselected conditions such as the ratio of gel to core particles. Onaverage the glass layer should be between 0.2 and 0.3 μm thick.

[0025] A preparation is particularly preferred which contains particleshaving a glass surface in which more than 75% by weight of theseparticles have a particle size between 2 and 15 μm. The proportion ofparticles with the defined particle size is particularly preferably morethan 90% by weight.

[0026] Magnetic core particles are particularly preferably used. Anadvantage of the preparation according to the invention is thatpreferably more than 95% by weight of the particles having a particlesize between 0.5 and 15 μm, preferably between 2 and 15 μm are magnetic.This means that the proportion of particles that do not contain cores isdrastically reduced in comparison to the known processes. This can berecognized by the fact that only a few non-magnetic particles arepresent. This means that it is practically no longer necessary toseparate the non-magnetic particles from the magnetic particles beforeusing the preparation in processes to purify nucleic acids. Thissimplifies the production process.

[0027] The preparation according to the invention can be additionallycharacterized in that preferably less than 50% of the particles have aparticle size of less than 2 μm. Consequently there is a substantialreduction in the non-magnetic fine fraction which has a relatively highproportion in particles of small sizes. Particularly preferably lessthan 2% of the particles have a particle size of less than 0.5 μm.

[0028] Preferably no more than 10% and particularly preferably between10 and 40% of the particles of the preparation have a particle size ofmore than 10 μm.

[0029] In addition to the particles according to the invention theinventive preparation can also contain other non-glass containingcomponents such as buffer substances or a suspending agent e.g. water oralcoholic solutions of water.

[0030] The glass layer of the particles of the inventive preparationpreferably contains between 2 and 6 mole %, particularly preferably 4mole % zinc oxide. This can be achieved by having an amount of zincoxide in the solid sol mass of this order of magnitude compared to theamounts of the other solid components. The proportion of zinc oxideincreases as the amount of boron oxide decreases especially when thepreparation is heated for long periods since boron oxide is alreadyvolatile under the production conditions.

[0031] Particles having a glass layer in which the proportion of zincoxide is between 2 and 6 mole % have proven to be particularly effectivefor purifying nucleic acids. The yield of nucleic acids was in somecases increased by 50% compared to the same glass layer without zincoxide.

[0032] Another subject matter of the invention is a process forproducing a preparation of particles having a core coated with a gellayer that contains less than 5% by weight particles without corescomprising the steps suspending core particles in a sol using a coreparticle preparation and spray drying the suspension to form a gelwherein the core particle preparation contains 75% by weight particleswith a particle size between 0.5 and 15 μm, preferably between 2 and 15μm.

[0033] Reference is made to the descriptions in the prior art withregard to carrying out the gel/sol process which is used by theproduction process according to the invention. The main differencebetween the invention and the prior art is the use of a particular coreparticle preparation which enables production of a preparationcontaining less than 5% by weight particles without cores. A process hasproven to be particularly advantageous in which firstly a sol isprepared from tetraalkyl orthosilicates, alkyl borates, aluminiumalcoholates and alkali alcoholates in ethanol and this mixture is heatedwith calcium. Subsequently the mixture is hydrolysed by adding water.Core particles are added in a solid form to the sol formed in thismanner and are suspended preferably with ultrasound. Subsequently thesuspension is atomized to form a gel in a spray drying process in whichthe nozzle is heated and in which mainly particles are formed containingbetween 1 and only a few core particles per particle (preferably lessthan 1% of the particles contain more than 10 core particles). The sprayproduct is subsequently heated in order to compress the gel to form aglass. Also in this case the addition of zinc oxide to the gel is veryadvantageous. The compression can be carried out at higher temperaturesthan with preparations to which zinc has not been added since thesoftening point of the glass that is formed is higher. This enablesorganic residues to be more easily expelled from the starting materials.

[0034] Since a preparation containing a very low proportion of particleswithout cores is formed in the process according to the invention, it isin general no longer necessary to subsequently fractionate particleswith and without cores.

[0035] The invention also concerns a process for purifying nucleic acidsby non-covalently binding nucleic acids from a sample to particleshaving a glass surface, removing non-bound sample components and elutingthe bound nucleic acids from the glass surface wherein a preparationaccording to the invention is used. The process is particularly simplewhen the particles are magnetic.

[0036] Processes for purifying nucleic acids with the aid of magneticparticles having a glass surface are described in WO 96/41811. Referenceis herewith made to the complete contents of this disclosure. Suitablesamples for the purification process according to the invention are inparticular clinical samples such as blood, serum, mouth rinse liquid,urine, cerebral fluid, sputum, stool, plasma, biopsy specimens or bonemarrow samples. Serum is a preferred sample material. In order to purifythe nucleic acids the sample, if required after lysis of cellularstructures that may be present and digestion of interfering samplecomponents, is admixed with the inventive preparation e.g. in the formof a certain amount of a particle suspension. After an incubation periodduring which the nucleic acids bind sequence-unspecifically to the glasssurface, the liquid together with the non-bound sample components isremoved and the particles are washed, if desired, in order to removeresidues. The nucleic acids which are still bound thereto are removedfrom the surface by elution with a liquid in which the nucleic acidsdissolve well. The resulting liquid can now be processed in any desiredmanner and in particular be used in amplification methods, e.g. PCR,since most of the enzyme inhibitors are separated during thepurification process.

[0037] If the particles are magnetic, it is particularly easy to removethe liquid from the particles with the nucleic acids since the particlescan be collected and held with the aid of a magnet while the liquid isremoved. If the particles are non-magnetic they can be separated fromthe liquid by filtration using a suitable filter.

[0038] The present invention is elucidated in more detail by thefollowing examples.

EXAMPLE 1

[0039] Sol for Preparing a Zinc-Free Layer(74SiO₂×15B₂O₃,×4K₂O×2CaO×5Al₂O₃)

[0040] 1750 ml tetraethyl orthosilicate (manufacturer: Wacker,Burghausen) is placed in a 5 litre round bottomed flask and thefollowing are added rapidly at room temperature while stirring (500rpm):

[0041] 541 ml triethyl borate (manufacturer: Aldrich, Steinheim)

[0042] 250 ml potassium methanolate (25% in methanol (manufacturer:Fluka, Deisenhofen))

[0043] 261 g aluminium sec. butylate (manufacturer: Aldrich, Steinheim)

[0044] 292 ml ethanol and

[0045] 8.49 g calcium (manufacturer: Fluka, Deisenhofen)

[0046] The mixture is subsequently heated to a strong reflux whilestirring. A mixture of altogether 583 ml ethanol and 233 ml water isadded dropwise for a period of 30 minutes. After cooling to <50° C. thesol is transferred to an open container and 1200 g of the pigmentIRIODIN 600 Black Mica (manufacturer: Merck, Darmstadt) is added. Aftercompletion of the pigment addition the sol is stirred for a further 1minute at 500 rpm and subsequently treated for 5 minutes withultrasound. After the ultrasonic treatment the sol-pigment mixture isstirred with a dissolver stirrer at ca. 500 rpm until the entire amountis consumed.

EXAMPLE 2

[0047] Preparation of Glass-Coated Pigment (MGP)

[0048] The spraying is carried out in a spray tower from the NubilosaCompany, Konstanz having a diameter of 0.75 m, a height of 2.5 m and anevaporation capacity (with reference to water) of 1-3 litres/hour. Theair intake temperature is 270° C., the outlet temperature is ca. 130° C.The flow rate of air is 7.2 m³/min. A two-fluid nozzle with a spraypressure of 2 bar is used for spraying. The delivery capacity of theball valve membrane pump is 60 g sol/min.

[0049] The spray product is captured in a cyclone, precompressed in airfor 1 hour at 250° C. and subsequently brought to a temperature of 675°C. in a nitrogen oven at a heating rate of 1 K/min, kept for one hour inthis oven and cooled to 300° C. Oxygen is added at 300° C., it is keptfor one hour and then cooled to room temperature. After cooling it issieved using a sieve having a mesh size of 50 μm to remove aggregatesthat may be present. This completes the preparation.

EXAMPLE 3

[0050] Sol for Preparing a Zinc Containing Layer(70.67SiO₂×14.33B₂O₃×4K₂O×2CaO×5Al₂O₃×4ZnO)

[0051] A sol containing zinc is prepared in the same manner asexample 1. For this the following weights of educts are added andtreated in a similar manner: 1258 ml tetraethyl orthosilicate(manufacturer: Wacker, Burghausen) 387 ml triethyl borate (manufacturer:Aldrich, Steinheim) 188 ml potassium methanolate 25% in methanol(manufacturer: Fluka, Deisenhofen) 196 g aluminium sec. butylate(manufacturer: Aldrich, Steinheim) 1285 ml ethanol 6.39 g calcium(manufacturer: Fluka, Deisenhofen) 58.5 g zinc acetate dehydrateddihydrate (manufacturer: Fluka, Deisenhofen)

[0052] After boiling under reflux, 178 ml H₂O together with 444 mlethanol are added dropwise within 30 minutes. After cooling 1200 gpigment is added. Otherwise refer to example 1.

EXAMPLE 4

[0053] Preparation of a Zinc-Containing Glass-Coated Pigment

[0054] The sol containing pigment from example 3 is processedanalogously to example 2. However, the compression temperature is 750°C.

EXAMPLE 5

[0055] Preparation of Zinc-Containing Glass-Coated Pigment Using aModified After-Treatment (MGP, Magnetic Glass Particles)

[0056] The sol containing pigment from example 3 is processedanalogously to example 2. However, the compression temperature is 750°C. and the temperature for treatment in oxygen is 200° C.

EXAMPLE 6

[0057] Determining the Yield of DNA or RNA Using Radioactive ³²P

[0058]³²P-labelled HIV gag RNA standard of 1.4 kb or ³²P-labelled lambdaamplicons of 3 kb is used to directly detect bound or non-bound DNA orRNA. Negative plasma (human) each containing 10⁹ copies is used as thesample.

[0059] Procedure for Sample Preparation

[0060] 500 μl negative plasma containing 10⁹ copies ³²P-labelled lambdaamplicons is placed in a 2 ml Eppendorf vessel. 480 μl bindingbuffer/proteinase K solution (5:1) is added by pipette, vortexed andincubated at 70° C. for 10 minutes. After cooling to room temperature,400 μl isopropanolic MGP suspension containing a total of 3 mg MGP isadded by pipette. Immediately afterwards it is mixed by vortexing. Thesample is then incubated for 15 minutes on a mixer e.g. thermomixer 5436from Eppendorf.

[0061] The MGP are concentrated by transferring the sample to a magneticseparator. After one minute the supernatant is completely removed bypipette.

[0062] 0.5 ml washing buffer is added by pipette to the MGPs. The sampleis vortexed and then transferred to a magnetic separator. Thesupernatant is removed by pipette after 1 minute. The washing procedureis repeated a further 2 times.

[0063] 200 ml elution buffer is added to the MGP. They are incubated for10 minutes at 80° C. on a thermomixer at 1400 rpm. The sample istransferred to a magnetic separator and after 1 minute the entire eluateis removed. The eluate is then transferred to a new vessel and measuredin a scintillation counter.

[0064] The yield can be determined from the ratio of the radioactivityof the eluate to the radioactivity of the sample before the purificationprocedure.

[0065] Results with MGPs with Different Coatings: DNA yield RNA yieldMGP of example 2 (without 44% 44% zinc) MGP of example 4 (with 62% 59%zinc) MGP of example 6 (with 66% 70% zinc, modified after- treatment

EXAMPLE 7

[0066] Black Mica (BM) as the Pigment Base (Reference Example)

[0067] A batch is manufactured according to example 1 in which thepigment is black mica (BM).

EXAMPLE 8

[0068] Microna Mat Black (MMB) as the Pigment Base

[0069] A batch is manufactured according to example 1 in which thepigment is MMB (microna mat black (manufacturer: Merck, Darmstadt).

EXAMPLE 9

[0070] Signal Level after Amplification in which the Samples have beenPrepared Using MGPs Containing Different Pigments

[0071] MGPs according to example 7 and 8 are used for the samplepreparation. Human plasma containing 100 copies/ml HCV viruses is usedas the sample. The eluate of the sample preparation is subjected to anamplification and the amplification result is detected by anelectrochemiluminescence method. In an additional experiment the samplewas human plasma containing 600 copies/ml HBV viruses. HCV ECL countsHBV ECL counts MGP according to 97000 25000 example 7 (BM) MGP accordingto 127000 43000 example 8 (MMB)

EXAMPLE 10

[0072] Carbonyl Iron Powder HQ as the Pigment

[0073] A zinc containing sol is prepared according to example 3 but onlywith 240 g sol.

[0074] After cooling 71 g carbonyl iron powder HQ (BASF, Ludwigs-hafen)with a particle size distribution of: 10%<0.5 μm, 50%<1.1 μm, 90%<2.2 μmis added, stirred for 1 minute at 500 rpm and subsequently treated withultrasound for 5 minutes. The sol is sprayed in a spray dryer (Buche190, Mini Spray Dryer). The nozzle temperature of the spray dryer is140° C.

[0075] The powder obtained is heated in air at 150° C. The heating rateis 1 K/min and the holding time is 1 hour. Subsequently the air in theoven is replaced by N₂, flushed several times and heated at 1 K/min to700° C., held for 1 hour, cooled to 200° C. at 1 K/min. Nitrogen isreplaced by air at 200° C. and held for 1 hour. It is then cooled toroom temperature. Aggregates that may have formed are removed by sievingwith a 50 μm sieve.

[0076] The yield is 62.4 g. The sieve losses are negligible. Aggregatesdo not occur.

EXAMPLE 11

[0077] Determining the Binding of RNA

[0078]³²P-labelled HIVgag standard of 1.4 kb is bound according toexample 6 to the particles of example 10. The radioactivity measurementyielded a binding of >80%.

EXAMPLE 12

[0079] Comparison of the Sedimentation Rates

[0080] 3 mg particles of example 10 are transferred into two Eppendorfvessels having a volume of 2 ml and suspended with 1.5 ml H₂O in eachcase.

[0081] The particles in vessel 1 are attracted to the vessel wall with amagnet and subsequently resuspended by shaking. The particles in vessel2 are shaken up at the same time.

[0082] Sedimentation in a gravitational field is observed visually. Nodifferences occurred.

1. Preparation containing particles having a glass surface, wherein morethan 75% by weight of these particles have a particle size between 0.5and 15 μm.
 2. Preparation as claimed in claim 1, wherein more than 95%by weight of the particles having a particle size between 0.5 and 15 μmare magnetic.
 3. Preparation as claimed in claims 1 or 2, wherein lessthan half of the particles have a particle size of less than 2 μm. 4.Preparation as claimed in one of the claims 1, 2 and 3, wherein lessthan 2% of the particles have a particle size of less than 0.5 μm. 5.Preparation as claimed in claim 2, wherein the magnetic particles have amagnetic core which is coated with glass.
 6. Preparation as claimed inclaim 1, wherein no more than 10% of these particles are particles witha particle size of more than 10 μm.
 7. Preparation as claimed in one ofthe previous claims, wherein the particles have a glass coat whichcontains between 2 and 6 mole % zinc oxide.
 8. Preparation as claimed inone of the previous claims, containing at least one core of a magneticmetal.
 9. Preparation as claimed in claim 8, wherein the core or coreshave a particle size of between 0.01 μm and 100 μm, particularlypreferably between 0.2 μm and 8 μm.
 10. Process for producing apreparation of particles having a core coated with a gel layercontaining less than 5% by weight particles without cores comprising thesteps suspending core particles in a sol using a core particlepreparation spray drying the suspension to form a gel, wherein the coreparticle preparation comprises 75% by weight particles having a particlesize between 0.5 and 15 μm.
 11. Process as claimed in claim 9, whereinthe sol contains zinc.
 12. Process for purifying nucleic acids bynon-covalently binding nucleic acids from a sample to particles having aglass surface, removing non-bound sample components and eluting thebound nucleic acids from the glass surface, wherein the sample iscontacted with a preparation as claimed in one of the claims 1 to 8 or15 to
 16. 13. Process as claimed in claim 11, wherein the particles aremagnetic and are held by a magnet while the sample components areremoved.
 14. Use of zinc oxide in glass layers generated by a sol/gelprocess to increase the binding capacity of the glass surface fornucleic acids.
 15. Process for producing a preparation of particleshaving a core coated with a gel layer containing less than 5% by weightparticles without cores comprising the steps suspending core particlesin a sol using a core particle preparation spray drying the suspensionto form a gel, compressing the gel to form a glass, wherein the coreparticle preparation comprises 75% by weight particles having a particlesize between 0.5 and 15 μm.